1. Field of the Invention
The present invention relates to a piston structure of a tilt apparatus for a marine propulsion device.
2. Description of the Related Art
Conventionally, there has been a tilt apparatus for a marine propulsion device structured such that a cylinder apparatus is interposed between a hull side and a propulsion unit side. The piston constituting the cylinder apparatus has a shock blow valve and a return valve. In this tilt apparatus, when an impact force in an extending direction is applied to the cylinder apparatus due to a collision with, for example, drift wood against the propulsion unit, the shock blow valve is opened at a set pressure so as to transfer a working fluid in a rod side oil chamber of the cylinder apparatus to a piston side oil chamber, thereby extending the piston rod and displacing the propulsion unit up. The return valve is opened due to pressure in the piston side oil chamber increased under operation of a dead load of the propulsion unit after absorbing the impact force, which returns the working oil mentioned above transferred through the piston side oil chamber to the rod side oil chamber, thereby returning the propulsion unit to its original position.
However, in the prior art, as shown in FIG. 9, when assembling a piston 2 in a piston rod 1, a ball 4 of a return valve is installed to a return hole 3 provided in the piston 2. Next the piston 2 is reversed so as to insert a ball, a valve seat and a valve spring of a shock blow valve (not shown) into a shock blow hole (not shown) provided in the piston 2. The piston 2 is attached to the piston rod 1 via a washer 5, and further fixed to the piston rod 1 via a washer 6 by a nut 7. Then, in order to prevent the ball 4 installed to the return hole 3 from falling down driving reversal of the piston 2 mentioned above, the structure is made such that a ball holding pin 8 is inserted to a horizontal hole crossing to the return hole 3 provided in the piston 2. Reference numeral 9 denotes an O-ring provided on an outer periphery of the piston 2.
In this case, in the prior art, in order to make the tilt apparatus compact, it is necessary to make the cylinder apparatus compact, so that it is necessary to make a diameter of the piston 2 small and a size thereof short.
The prior art has the following problems.
(1) Both the shock blow valve and the return valve are arranged with the piston 2. The O-ring 9 is provided on the outer periphery of the piston 2. Since the piston 2 is provided with the large-diameter shock blow hole to which the ball, the valve seat and the valve spring constituting the shock blow valve are inserted, it is necessary to prevent the diameter of the piston 2 from being increased. An O-ring groove 9A in a fixed thickness portion in an outer peripheral side of the piston 2 is formed so that the O-ring groove 9A is required to be formed in a thick portion above the shock blow valve (in a side of the rod side oil chamber). Then, unless the ball holding pin 8 is set further above the O-ring groove 9A (in a side of the rod side oil chamber), the oil in the piston side oil chamber leaks to the rod side oil chamber from a gap of the ball holding pin 8, whereby it is impossible to keep the piston position. As mentioned above, when the O-ring groove 9A is provided above the shock blow valve of the piston 2 and the ball holding pin 8 is provided further above the O-ring groove 9A, it is impossible to prevent the piston 2 from being extended in an axial direction, and it is impossible to make the piston 2 short.
(2) When the horizontal hole for the ball holding pin 8 is provided in the piston 2, it is necessary to apply a drilling process to the horizontal hole, and a spot facing and flattening process to a worked surface of the horizontal hole, whereby the number of processes is increased.
(3) The operation of inserting the ball holding pin 8 to the horizontal hole of the piston 2 is simple but can not be rapidly performed, so that the efficiency of assembly is deteriorated. The position of the horizontal hole must be determined by the operator with each new piece.
An object of the present invention is to provide a piston structure of a tilt apparatus for a marine propulsion device in which a piston is compact while preventing a return valve from falling off during assembly of a valve body. A further object is to improve workability and ease of assembly of the piston.
In accordance with the present invention, there is provided a piston structure of a tilt apparatus for a marine propulsion device structured such that a cylinder apparatus is installed between a hull side and a propulsion unit side. A piston constituting the cylinder apparatus has a shock blow valve and a return valve, wherein the return valve attaches a valve body to a return hole provided in the piston, and valve body holding means is provided in the return hole.